Is Gastric Flu caused by food poisoning or in the air?

Hi there! Any doctors out there can help me please. Doc says I have gastric flu, high temperature one min the freezing cold the next, nausea, the runs, pain in gut. He says I am infectious and not to go out for a few days and I won't, but........looking on the net it says it is caught eating or drinking something dodgy! So how can I be infectious? And how did I actually catch it? And why hasn't anyone else in my family got it yet!! Thanks :)